This invention relates to a method for preparing an insoluble hydrolysable tannin which can adsorb heavy metal elements including actinides, such as, uranium, thorium, transuranium elements, and the like, and to the insoluble hydrolysate tannin obtained thereby. It also relates to a method for treating a waste liquid containing the heavy metal element in which the heavy metal is adsorbed and separated by using the insoluble tannin.